Metal structures have conventionally been used to create fittings, provide bearing surfaces and attachment means because metal is readily machine-able and typically more easily inspected as compared to fiber-reinforced composite structures. Metallic fittings are used in extensively in aircraft structures as interfaces between structural components, such as between a landing gear door and an aircraft fuselage. Metallic fittings take the form of hinges, structural splices, component attach points, and numerous other applications. Therefore, methods for using fittings in any structural application are important.
Structural composite materials are made from high tensile strength fibers and resin. The proportions of fibers and resins are carefully controlled to maximize the effective tensile and compressive strengths. However, even the hardest, toughest, and strongest composite materials are considered “soft” when compared to high strength metals, and have poor properties when machined or drilled. Specifically, drilling a fastener hole cuts the fibers within the structure and, therefore, significantly weakens it. Therefore, attaching a composite component to a metallic fitting or vice-versa is an engineering art that requires considerable skill.
The aircraft industry has captured these methods in numerous design guides and manuals. In general, two methods of attaching composite materials to metallic fittings are in use. One method for integrating a mechanical feature in a composite structure is based on a drilled and assembled fastener pattern. The patterns, to the extent possible, are optimized based on the shape of the pattern, number of fasteners, choice of fasteners, washers, nuts, bearing plates, torque, interference, and other variables. Regardless of hardware (nuts, bolts, washers, etc.), mechanical fastening is limited by its inherent process of cutting fibers (drilling and countersinking a hole), and forcing mating surfaces together—thereby crushing (and potentially exceeding the compressive strength of the composite material. It is not uncommon for bolts to pull through composite materials because joint loads exceed capability. The thicknesses of composite material should compensate for the cut fibers and low compressive strength, which typically results in additional composite material, and thus additional weight. This extra material is considered parasitic weight that is theoretically unnecessary, except for its function at the joint. This parasitic weight can total up to thirty percent of a composite component's weight.
Another method for integrating a mechanical feature into a composite structure is referred to as chemical bonding. Some examples are chemical bonding may include chemical etching and preparation of the surface (phosphoric acid anodize, chromic acid anodize, sol-gel treatments); mechanical preparation of the surface (splines, knurling, sanding, cross hatching); mechanical locking configurations (machined features that mechanically lock and bond a fitting onto the composite article); and load dropping features (ply drop offs that minimize load concentrations).
Although it is theoretically possible to bond a metallic fitting to a composite article, either during or after the composite article manufacturing process, aircraft materials and process experts rarely allow those bonds to be considered in structural applications. In spite of the disallowance of the structural bond as a load carrying mechanism, it is somewhat common to see bonded fittings in composite articles. Usually, when bonded joints like this are accomplished, “chicken” fasteners are used to prevent peeling and/or provide alternate load paths to the bonded joint. It is rare that this type of joint would result in any weight advantage compared to a traditional fastened joint described above.
The term “failsafe” is an aircraft engineering term that, in essence, means: “if, for some reason, a structural load path fails, such as a bond line between a fitting and a component, an alternate load path between the fitting and the component will carry the intended design loads so there is no loss of function.” As noted above, fittings are sometimes machined to have mechanical locking features that, when used in combination with bonding methods, results in the composite part being locked onto the fitting. It should be noted that when this method is employed, the fitting is likely to be inside of the composite article, and the materials wrapped around its exterior. Depending on configuration, this approach can result in a failsafe design. However, this approach has very limited configurations that are useful since most are circular or rectangular by necessity. And, to create these shapes, external pressure has to be applied during the manufacturing process.